As per the conventional thread cutting device of a sewing machine, there are many prior arts, for example, TW Appl. No. 85203813, 86218218, 89207548, 89214516, 89214905, 91218393, 92129042, 94204676 and 95200698, which all use a linear actuator to actuate a connecting rod mechanism to make a thread hooking member stretch out or retract, however such mechanism has the following shortcomings, which are:
1. The whole structure is too complicated and large: the prior art used a linear actuator to control a thread hooking member back and forth, because the linear distance of the actuator is limited, therefore it must have a lever arm that the effort arm is a lot shorter than the load arm to be an active component to amplify the distance of the movement of the thread hooking member, so the distance for the thread hooking member moving back and forth can be satisfied, this type of structure has a large size due to the length of the lever arm, it is hard to comply with the necessary of miniaturization and simplification.
2. The mistake of the linear actuator is easy to get amplified: because the lever arm amplifies the moving distance, therefore any of the components has a mistake which will be amplified due to the long lever arm, which causes the thread hooking member short moving distance or over moving distance, the thread hooking member would be unable to cut the needle thread or over stretched then hit the corresponding components or structure and get damaged, so it needs to be improved.
3. Force-consuming structure is not economical: because the load arm is longer than the effort arm, which is a force-consuming structure that will consume improperly electricity and the actuator is easy to get damaged, therefore it needs to be improved.